Transmission capacity of an optical transmission system that uses the wavelength division multiplexing (WDM) technology has increased in recent years. For example, standardization, including optical channel transport unit 4 (OTU4), which transfers a client signal in Ethernet (registered mark) of a transmission speed per channel of the 100 Gbit/s class, is proceeding.
An example of a developed optical modulation scheme for transmitting a 100 Gbit/s signal over a long distance is a dual polarization quadrature phase shift keying (DP-QPSK) modulation and demodulation technique, which performs four-level phase modulation on orthogonal polarized waves propagating through an optical fiber.
To address a further increase in transmission capacity, application of a multilevel modulation scheme of four or more levels, such as 16 quadrature amplitude modulation (16QAM) is being pursued.
Also, an example of the related technique is adjustment of the degree of optical modulation on the basis of a transmission error rate as explained in, for example, Japanese Patent No. 3940083.
An example of the apparatus used in generating optical signals with a predetermined frequency spacing is an interleaver. An interleaver has the function of changing a frequency spacing (grid spacing) of optical signals and separating the wavelengths of (interleaving) the signals for individual optical modulators. Unfortunately, the interleaver can interleave signals only in a fixed band range.
As in an example case in which optical signals with 50 GHz (corresponding to the wavelength 0.4 nm) spacing are interleaved with optical signals with 200 GHz (corresponding to the wavelength 1.6 nm), interleaving is allowed only in a fixed band range, and control for interleaving in a bandwidth variable manner has not been established.
Thus, optical transmission in a plurality of optical modulation schemes with different frequency spacings has to prepare a plurality of interleavers that can deal with the optical modulation schemes and carrier frequency spacings, and this raises a problem of increasing the system scale, power consumption, cost, and other factors.